In a gas combustion type driving tool, as indicated in Patent Document 1, mixture gas obtained by stirring and mixing combustible gas and air together in a combustion chamber by a fan is ignited by sparks from an ignition plug and explosively combusted, and a driving piston is driven by gas pressure of this combustion gas to drive fasteners such as nails, screws, or the like. In such the gas combustion type driving tool, a combustion residue from additives of the combustion gas supplied in the combustion chamber can adhere to a center electrode of the ignition plug. The combustion residue adhering  to the center electrode of the ignition plug accumulates gradually on the leading end of the center electrode, which becomes a large cause to invite poor ignition of the ignition plug.
In the gas combustion driving tool, in order to return surely the driving piston after driving the fasteners to the initial position, increase in quantity over the most suitable quantity of gas density is performed. Hereby, even in the usual combustion time, the many are produced. The residue adheres to a wall portion of the combustion chamber, the ignition plug, and the like. In particular, an attachment position of the ignition plug in the combustion chamber is a position at which the wind of a stirring fan is difficult to arrive. Therefore, the residue is easy to adhere to the ignition plug. The ignition plug is disposed facedown at the upper portion of the combustion chamber, and further the residue adhering to the center electrode of the ignition plug is comparatively high in viscosity. Therefore, while the combustion is repeated many times, the residue flows down along the outer surface of the center electrode little by little and arrives at the leading end of the center electrode. The residue which has arrived at the leading end, without dropping, stays at the leading end for a while. Since a residue which flow down  sequentially from the upside stick to the residue which has stayed at the leading end, the residue grows gradually. In result, the poor ignition is caused.
In order to solve the above-mentioned disadvantage, an improved ignition plug has been disclosed in Patent Document 2. In an ignition plug in the Patent Document 2, a free end of a spark unit electrode (electrode of the ignition plug), that is, a spark ejected leading end portion (electrode leading end portion which ejects sparks) protrudes positively to the outside from the lower surface of a boss to which the spark unit electrode is attached. Hereby, the improvement is made so that a recess portion or a pocket portion is not formed around the free end of the electrode, whereby oil or dust is not accumulated around the free end of the electrode, with the result that the electrode is protected and trouble such as the poor ignition is prevented.
Patent Document 1: JP-B-04-048589
Patent Document 2: JP-A-2003-176773
However, the countermeasure for protecting the electrode in the above Patent Document 1 is taken for protection of the electrode from the oil or dust accumulated in the recess portion or the pocket portion, and there is particularly no electrode  protecting countermeasure from a view of preventing a residue from adhering to the protruded electrode leading end portion. By such the known electrode protecting countermeasure, it is not possible at all to solve the occurrence of trouble such as poor ignition due to adhesion of the residue to the ignition plug in the gas combustion type driving tool under the above circumstances.